Conduction, Convection, &
Radiation
Vanderbilt Student Volunteers for
Science
Fall 2013
I. Introduction- What is temperature?
What is heat?
• Ask: what is temperature?
– scientific measure of how hot or cold something is
– measured with a thermometer
– measure of the average kinetic energy of the
molecules in a substance.
• Ask: What happens when we add ice to hot
tea?
The ice melts. Energy from tea is transferred to the
ice. This transfer of energy is called heat.
2. Liquid Crystal Temperature Sensors
• Ask students if they have seen objects that change color
with temperature. (mood rings, strip thermometers..)
• Discuss the RULES for using the temperature sensors:
– The sensor must not be placed in the sun or near a
heat source
– Always hold the strips using the clear laminated part
– Do NOT touch the liquid crystal unless instructed to do
so.
• Hand out liquid crystal temperature sensors to each
pair of students.
2A. Experiment
• Tell students to place the
temperature sensor on the
white foam pad, black side up.
• Tell 1 student/pair to place 3
fingertips on the liquid crystal
sensor for 15 sec. Record what
happens.
• Observations:
– color of the sensor changes.
– color changes spread out
from the center of the finger
and changes color as it
spreads.
2. Discussion
• Ask : What color is in the center of the finger print? At
the outer edge? What color fades first? Last?
What color =coolest temp? – Black
What color = warmest temp? – Blue
• Order from cooler to warmer temperatures? Black,
yellow, red, green, blue.
• Explanation: The surrounding area’s colors were
produced as a result of conduction of heat through the
crystal.
• Place strip back on the desk top to return to room
temp.
2B. How do Liquid Crystals work?
• Explain to students that liquid crystals are sensitive to temperature
and will separate visible light into different colors according to the
temperature of the crystal. Black is the coolest color.
• Where does the thermal energy come from to change the liquid
crystal color?
– Answer: Your skin temperature is higher than room temperature. Heat
flows from your fingers into the object.
• Where does the energy go when the color changes back to its
colder temperature?
– Answer: It is transferred to the desk or Styrofoam pad.
Heat flows from the warmer to the cold.
3. How is energy transferred?
• Thermal energy is transferred by heat.
– Heat always flows from a hotter object to one that is
cooler. In this case, your body is warmer than room
temperature and heat flows from your finger to the sensor.
• Ask students if the know the different ways thermal
energy is transferred?
Conduction, convection, or radiation.
• Tell students we are going to “see” conduction,
convection and radiation by using the liquid crystal
sensors.
3. How is energy transferred?
A. Radiation
• Radiation is the transfer of energy by
electromagnetic waves.
• Ask: Can you name some sources that transfer
heat by radiation?
– The sun, a fire, bar heater, incandescent light bulb ….
• Radiation does not rely upon any contact
between the heat source and the heated object.
We can feel heat from the Sun or a fire even
though we are not touching them.
3. Demonstration - Visualizing
Radiation
A VSVS member holds the large liquid
crystal sensor up. Note the color.
Hold the lamp in a vertical position, turn
it on and hold the large sensor vertically,
about 10 inches away.
Wait until the sensor changes color, then
remove the light and observe changes.
Optional Demonstration: Visualizing Sun
Radiation (if the sun is shining into the
classroom)
Now hold the sensor in the sunlight and
note the color changes.
Point out that there is no contact
between the lamp and the sensor.
3. Experiment – Visualizing
Convection
Define convection and ask: Can you name
some sources that transfer heat by
convection?
Heating water, hot air balloons…
Experiment:
1. Show how to activate the hot pack and
place on the white foam board.
2. Tell students to hold their hands above it, to
the side, and below it to feel convection
3. Hold the liquid crystal sensor 2 cm above
the heat pack (DO NOT TOUCH IT) and note
color changes.
Explanation: When the heat pack warms the
air next to it (through conduction). The warm
air molecules move more quickly, becomes
less dense, and rises. The sensor changes
color.
C. Conduction
• Define: Conduction is the transfer of thermal
energy between objects that are touching.
• Ask: Can you name a good and poor thermal
conductor?
Metals are usually good conductors.
Foam boards are not.
Experiment: Visualizing Conduction
•
•
Hand out the thermal conductivity boards.
Ask students if they can identify the 3
different materials?
Copper, wood and iron.
•
Tell students to place the heat pack at the
lowest edge of the 3 exposed prongs.
– Wait a few minutes and observe changes
in liquid crystal colors.
•
Record the observations.
– The color change starts nearest the heat
pad, travels up the materials.
– The rates are different for the 3
materials.
•
What metal had the fastest changing
temperature sensor? Copper
– Slowest? Wood
– What is the best thermal conductor?
Copper
Experiment 2. Observing Ice melting
Distribute pairs of black blocks
Place some ice in the middle of each block and tell
students to observe. Do not tell them what the
blocks are made of.
Pass out the plastic bags with wood, styrofoam
and metal squares and thermometer strips.
Spread on long piece of foam board.
Tell the students to:
1. Place a palm on top of each block (no more than
1 sec).
2. Put the blocks in order from coolest to warmest,
record results on the board (aluminum, wood,
foam).
3. Set aside the squares so that they can return to
room temperature.
Experiment 3. Ice Melting on 3
Different Materials.
•
Show the students a strip
thermometer and explain that it is
made of the same liquid crystal
material, with a temperature scale
added.
– The dark blue color denotes
the room temperature
• Tell students to place a strip on
wood block and record
temperature. Repeat with the
Styrofoam and metal blocks.
• Ask students if the temperatures of
the blocks are different.
(Should be no significant difference
within a group)
Experiment 3. Ice Melting on 3
Different Materials cont.
• Ask: What will happen to ice if
placed on the blocks?
• Tell students to place a small
piece of ice in the middle of each
block and record results.
The ice on the metal will
melt in just a few seconds.
• Wipe ice off the metal block and
measure temperature again.
• Ask: Why did the ice on the metal
melted faster?
Metal is a good conductor, it
transfers heat from it to the
ice faster than the other 2
materials.
Results
Ask: Why did the metal square felt cooler than the others, when the actual
temperature is the same?
The cold square is CONDUCTING heat away from hand so it is actually your
hand that is cooling.
Ask students which object is the best conductor?
The metal square is the best conductor.
Ask students which object would be a good insulator?
Styrofoam.
CLEAN UP
•
•
•
Wipe off wet surfaces.
Place all materials back in kit. Discard leftover ice at the school if possible.
Return kit immediately. It is needed by another group the following day.
Radiation Extension
Radiation extension:
A VSVS member can cast a shadow on one side of the large sensor
with a book. This blocks the radiation reaching half the sensor .
Expose the sensor to the lamp or to the sun for a few minutes and
note the color changes.
Students will see that the sun’s radiation does not travel
through the book.
Ask students how this could relate to weather on a cloudy
day?
The sensor can also be held at an angle to the lamp’s or sun’s rays
rather that facing it directly. Note the sensor warms up slower.
Ask students how this can relate to the seasons.